______,Grease 13 x 9 baking dish.,Cut potatoes into thin slices.,Layer potatoes in prepared dish.,Top with cheeses.,Melt butter in medium saucepan over medium heat.,Stir in flour, cook 1 minute.,Stir in milk, mustard, salt and pepper.,Bring to a boil.,Reduce heat and cook, stirring constantly, until mixture thicken.,Pour milk mixture over cheese and potatoes.,Cover dish with foil.,Bake 30 minutes.,Remove foil and bake 15 to 20 minutes or until potatoes are tender and top is brown.,Remove from oven, let stand 10 minutes before serving.